The present invention relates to an improvement of heat resistance of a polyparaxylylene derivative formed from a (2.2)-paracyclophane compound by a chemical vapor deposition method.
When a (2.2)-paracyclophane compound expressed by a below-described general formula 2 is heated to 600 to 700° C., the compound is readily decomposed due to the feature of its structure as shown by a below-described reaction formula I to become a xylylene radical, which is polymerized on the surface of a substance to form a coating film composed of polyparaxylylene.
According to this coating method, a conformal coating can be applied to objects to be coated which have various kinds of configurations. Since the film formed by this method is excellent in its gas barrier characteristics, insulating characteristics or a chemical resistance, the film is widely employed for coating electronic parts, space or aircraft parts or medical devices therewith.
(In the formula 2, X1 and X2 represent hydrogen, lower alkyl or halogen. X1 and X2 may be the same or different.)

The (2.2)-paracyclophane compound that is currently industrially used as a film material includes (2.2)-paracyclophane (in the general formula 2, X1 and X2=hydrogen), dichloro-(2.2)-paracyclophane (in the general formula 2, X1=hydrogen and X2=chlorine) and tetrachloro-(2.2)-paracyclophane (in the general formula 2, X1 and X2=chlorine). From the (2.2)-paracyclophane, polyparaxylylene is produced. From the dichloro-(2.2)-paracyclophane, polymonochloroparaxylylene is produced. From the tetrachloro-(2.2)-paracyclophane, polydichloroparaxylylene is produced. In the formula, n designates a degree of polymerization.
As the rate of commercial use of these materials, the dichloro-(2.2)-paracyclophane occupies about 90% as much as the entire part in view of the physical properties of a polyparaxylylene film and the easiness of coating.
However, in the field of a semiconductor and electronic parts, a coating material is strongly apt to reject chlorine, and accordingly, a use of coating by employing the (2.2)-paracyclophane including no chlorine has been liable to increase. Further, a quantity of use of the tetrachloro-(2.2)-paracyclophane is extremely low owing to below-described reasons.
Though this coating film has the very excellent performances as described above, the film has one serious problem in its use. This problem results from a fact that a methylene group having a structure of —CH2—CH2— for connecting benzene nucleuses in a polyparaxylylene structure is ordinarily easily oxidized. Further, the oxidation is accelerated due to the rise of temperature.
Accordingly, the coating film can be used in a high temperature area under an environment having no oxygen such as in inert gas or vacuum, however, the coating film has a limitation in view of temperature in its use under an environment having oxygen.
In this case, a difference arises in a heat resistance depending on the kinds or the number of substituents. In this connection, the maximum working temperature of the polyparaxylylene film having no substituent is said to be 100° C. and the maximum working temperature of the polymonochloroparaxylylene film having one chlorine substituent in the benzene nucleus is said to be 120° C. On the other hand, the maximum working temperature of the polydichloroparaxylylene film having two chlorine substituents in the benzene nucleus is said to be 140° C. and has a considerably high heat resistance and the polydichloroparaxylylene is located at a high heat resistant grade in the polyparaxylylene film.
In fact, owing to this heat resistance, an attempt has been made that the tetrachloro-(2.2)-paracyclophane is used as a coating material of a ferrite core. However, since the polymerizing characteristics of a biradical produced by the thermal decomposition of the tetrachloro-(2.2)-paracyclophane are too strong and vapor thereof is heavy, the biradical is hardly diffused and is polymerized in the vicinity of an inlet of a coating chamber. Thus, the object to be coated with the coating material is hardly efficiently and uniformly coated with the polydichloroparaxylylene film. Therefore, at present, such an investigation has not been made. Only a small quantity of these materials is employed for a special use.
Various attempts for improving the heat resistance of the polyparaxylylene film have been made as shown in, for instance, U.S. Pat. Nos. 4,176,209, 5,267,390 and 5,270,082, and an attempt is proposed that an antioxidant is mixed with paracyclophane and the mixture is used to form a film.
However, techniques disclosed in these documents do not adequately solve the problem, and further, may possibly deteriorate film characteristics.
On the other hand, octafluoro-(2.2)-paracyclophane having a structure shown in a below-described formula 5 that all hydrogens of four methylene groups of (2.2)-paracyclophane are replaced by fluorines is used as the coating material.

A polytetrafluoroparaxylylene film (formula 6) shown by a below-described formula 6 that is produced by a chemical vapor deposition of the coating material has been known to have an excellent heat resistance. As for a method for producing the compound (formula 6), some examinations have been made as shown in, for instance U.S. Pat. Nos. 3,268,599, 3,274,267, 3,297,591, Japanese Patent Application Laid-Open No. hei 5-255149, Japanese Patent Application Laid-Open No. 2001-213818 and Japanese Patent Application Laid-Open No. 2001-226300.
(in the formula, n represents a degree of polymerization.)
Further, the inventors of the present invention clarified that a poly-α, α-difluoroparaxylylene film (shown by a below-described formula 8) that is produced by chemical vapor deposition using tetrafluoro-(2.2)-paracyclophane (shown in a below-described formula 7 that is produced by replacing hydrogens of two methylene groups of four methylene groups of (2.2)-paracyclophane located in a diagonal line by fluorines) as the coating material had the excellent heat resistance (Japanese Patent Application Laid-Open No. hei 9-25252 and Japanese Patent Application Laid-Open No. hei 10-195181).
(In the formula, n represents a degree of polymerization.)
However, since the two coating materials need a fluorination process during producing the coating materials, a production cost cannot be avoided from increasing. Further, since the polymerization speed of a biradical produced in a thermal decomposition during a polymerization is very low, a special operation is required such as cooling an object to be coated with the coating material to accelerate the polymerization.
Accordingly, under these circumstances, the coating materials are hardly used for coating an inexpensive generalized product.